120 MOLLUSCA FROM THE GREAT OOLITE. 



Our specimens agree more nearly with the figures of Agassiz than with those of 

 Goldfuss ; in the latter, the posterior side is not so much raised, so that the hinge margin 

 slopes downwards in a manner similar to that of the anterior border, and the costal angle 

 is not directed obliquely backwards ; so that, judging from the figure alone, it might be 

 regarded as a distinct species. Compared with Goniomya v. — scripta, our shell has much 

 less prominent umbones, and the entire figure of the shell is more elongated or sub-cylin- 

 drical, the umbones being likewise more anterior ; the posterior side of the shell is more 

 lengthened, its superior margin being nearly horizontal. It is comparatively rare ; we have 

 obtained it in thin layers of pale or buff-coloured argillaceous limestone, about 100 feet 

 above the fullers' earth, also in a much lower position, in a similar description of rock ; but 

 the genus has not been found in the shelly beds of the formation. Height and diameter 

 through both valves equal, or half the longitudinal diameter. 



Locality. Minchinhampton. 



Goniomya he.micostata. Tab. XII, fig. '6. 



Testa ovato-elongatd, convexd ; umbonibus ante-medianis magnis subcowpressis, margine 

 antico oblique-declivi, postico subhorizontali, concavo-hiante ; superficie in medio oblique, 

 drpresso, costis crebris biangulatis aid trapeziformis instruct is; costis inferioribus 

 evanescetitibus. 



Shell ovate, elongated, convex, gaping posteriorly with a considerable aperture; 

 umbones anterior to the middle of the shell, large, elevated, but somewhat compressed; 

 anterior border sloping obliquely downwards, posterior border lengthened nearly horizontal 

 and concave ; the middle portion of the shell has a wide depression which passes from the 

 umbo directed slightly backwards and vanishing towards the inferior border ; the superior 

 and middle portion of the surface has numerous closely arranged costse directed upon each 

 side obliquely downwards towards the other, but connected with it by a horizontal straight 

 oosta ; the lower half of the shell and the two extremities are altogether smooth. Outer 

 or granulated layer of the test unknown. 



A single well-preserved cast with the valves in contiguity is our only authority. The 

 several features of this remarkable species clearly separate it from any other of the British 

 Goniomyae, the general figure with its elevated broad umbones, concave superior border, 

 gaping and slightly reflected posterior extremity combined with the wide mesial depression, 

 present no inconsiderable resemblance to a diminished figure of the great Panopcea 

 Aldrovandi ; the trapeziform direction of the costse is governed by the mesial depression, 

 and exists in those species only of the Goniomya3 in which this depression is well marked, 

 thus in Pholadomya trapezina, Buv., Lutraria trapezicostata, Pusch., and Goniomya inflata, 

 Ag. the horizontal costse extend, with the depression, even to the lower border of the valve ; 

 in the present species they extend, with the depression, about half the depth of the valve, 

 and in others, such as G. Dubois, Ag. G. v. — scripta, and G. littcrata, the depression and 



